1. Field of the Invention
The present invention relates to a brake control system for a vehicle, which is adapted to increase the braking force applied to wheels automatically, if necessary, when a brake pedal is depressed.
2. Description of the Related Arts
According to a known brake-assist control system, in the case where a vehicle driver depressed a brake pedal rapidly or deeply when driving the vehicle, the braking force applied to each wheel is increased so as to assist the driver to apply the braking force to the wheel. The conventional brake-assist control system is usually adapted to control a boosting power of a vacuum booster. For example, a Japanese Patent Laid-open Publication No.8-230634 discloses a system for performing a brake-assist control by means of a hydraulic pressure pump for use in an anti-skid control system. In that publication, a method and apparatus for controlling the anti-lock control system or a traction control system are proposed to save the power of the vacuum booster fully or partially. Also, in that publication, a dual hydraulic pressure circuit system is employed between a master cylinder and wheel brake cylinders, and a return pump is disposed in a passage in each hydraulic pressure circuit, while other structure is not clear, due to its insufficient description.
According to the brake system which divides the hydraulic pressure circuit between the master cylinder and the wheel brake cylinder into two hydraulic pressure circuits, and disposes the pressure pump in each circuit to be used as an auxiliary pressure source, the hydraulic braking pressure in each circuit is varied, because the characteristic of one of the pumps is different from that of the other pump. Therefore, especially in the diagonal hydraulic pressure circuit (X-circuit), the braking forces applied to the left and right wheels are different from each other, so that the stability of the vehicle might be deteriorated. In order to solve this problem, an input valve and an output valve as disclosed in that publication may be controlled with respect to each wheel. However, those valves are to be controlled so as to reduce the variation of the auxiliary pressure in addition to the control of the input valves and output valves used in the anti-lock control, so that noise caused in controlling the operation of those valves shall not be neglected.